In industrial solvent extraction processes, organic entrainment, solids and/or other contaminants must typically be separated from an aqueous phase to permit the subsequent recovery of valuable materials. The efficiency with which these components are removed from the aqueous phase directly affects the quality of the final product, as well as the costs associated with operating the system. For example, in the extraction of uranium from uranium ore by the use of solvents, the presence of contaminants within the system, particularly organic entrainment and solid particles, results in a contaminated final product having a reduced value.
In processes using solvents to extract copper, the presence of organic entrainment in the aqueous phase can cause so-called "burned" cathodes, thereby affecting the marketability of the solution produced by reaction at the cathodes. Moreover, such organic residues, when present in the aqueous phase, also result in increased corrosion of anodes constructed of lead based alloys, which in turn leads to accelerated contamination of the final cathodes with lead. The quality of the cathodes may be so drastically affected by this contamination that they no longer satisfy international norms and requirements for marketing and sale. Furthermore, the presence of such organic entrainment additionally results in greater contamination of the electrolyte vessel because of the high volatility of the organic entrainment at operating temperatures. These residues also increase the risks of fire in the electrolyte cells due to the short circuits typically generated in electrolytic operations.
The presence of solid particles in the aqueous phase can also have a substantial impact on the efficiency of the extraction process and thus these materials directly affect the cost of production. Such solid particles are normally associated with nodulation problems in the cathodes, which must be frequently rejected for this reason. In addition, higher levels of suspended solids involve greater flocculent requirements. Moreover, the solid particles also may form deposits on the surfaces of heat exchangers and thereby significantly diminish heat transfer efficiency. Further, when the solid particles are transferred to an electrolyte having greater acidity, harmful chemicals such as chlorine, manganese, and colloidal silica, may be generated by dissolution therein.
It is also important to emphasize that such solids constitute an additional source of contamination along with organic residues, due to their high absorption capacity. Returning these solids to the solvent extraction process by way of the spent electrolyte can contribute to a greater formation of "crud" in re-extraction stages, and consequently greater wastes and losses of organic reagent having a very high unit cost.
The term "crud" as used herein is commonly understood in the mining industry to refer to a close mixture of organic and aqueous solutions, together with a plurality of extremely fine particles which can be either organic or inorganic in nature. In some cases, the mixture also contains air distributed in a fine dispersion of bubbles. The mixture is present as an emulsified system of small drops of organic material suspended in an aqueous matrix, with the solid particles being distributed at the organic/aqueous interphase. The solids act as a bond to stabilize the mixture.
Finally, in some operations that use techniques of in-situ leaching or leaching in piles, solids and organic residues can clog the solution sprayers, which causes operating problems. In processes where copper sulfate is to be recovered from electrolytes by solvent extraction, the organic entrainment is trapped by the crystals, resulting in contamination which limits the product's marketability. Similar situations can also result in other solvent extraction applications during hydrometallurgical processing of commonly encountered metals.
Another very important aspect affecting operating costs in solvent extraction processes is the loss of organic reagent which becomes trapped within refinery solutions. By way of example, in a plant for the solvent extraction of copper that processes 1,160 m.sup.3 /hour of feed solutions and a volume of electrolyte of 600 m.sup.3 /hour, using an organic reagent at 31.degree. /v, losses of only 10 ppm of organic phase in these solutions can involve costs close to $600,000 per year. If losses of the organic phase increased to 40 ppm, the costs associated with such losses would increase to approximately 2.64 million dollars per year.
Hydrometallurgists have therefore long been attempting to develop methodologies for removing these organic entrainment, solid particles and other suspended contaminants from aqueous extraction phases. Techniques typically used in the prior art for this purpose include passing the electrolyte through one or more of the following: sand and anthracite pressure filters, columns containing activated carbon, centrifuges, and after-settlers. Another conventional approach for removing organic entrainment and solid residues from such aqueous solutions includes the use of a scavenger circuit at the electrowinning plant. These alternatives have certain drawbacks, however, since they generally require high investment and operating costs. In some specific cases, they also suffer from low efficiency, and for this reason they are used only to complement the final filtration process.
The use of sand and anthracite pressure filters, such as the well known Degremont filters, has been found to be a good alternative in some respects. However, a substantial drawback to the use of such filters is that they require a significant capital investment. By way of example, using these filters for the filtration of 100 m.sup.3 /hour of electrolyte requires an investment approaching $300,000. A sophisticated system of instruments and controls is also required for the use of such filters, thereby further increasing both the capital and operating costs. Another disadvantage associated with these filters is that, as a result of abrasion, the sand and anthracite therein have to be periodically replaced due to the loss of fine material from the filter, particularly due to back wash operations. Still another disadvantage is that, due to the high degree of sophistication of the associated equipment, costs for maintenance and spare parts are very significant. Further, the efficiency with which organic entrainment are removed by these filters is riot stable over time. Instead, this efficiency diminishes gradually, becoming critical when the levels of the residues in the feed line increase abruptly as a result of some operating problem. Yet another disadvantage is that the fine material clogs the small openings in the filter, thus affecting its operation.
Still other disadvantages are associated with the use of such anthracite and sand filters as discussed above. These filters must, for example, be subjected to a back wash step, normally after about every 16 hours of use, consequently consuming large quantities of air and water and leading to a significant amount of "down time" for the filters. Also, when these filters are partially emptied, part of the purified electrolyte containing the valuable chemical is lost. Further, because of the geometry of the filters, at the end of the backwash a significant quantity of organic phase always remains in the upper part of the filter. Finally, maintenance times for these filters are excessively long as a result of having to remove and then replace a solid filtering bed having a high specific weight.
The use of activated carbon columns is a further alternative. However, this technique has several serious limitations as set forth below:
1. The necessary equipment requires a substantial investment; PA1 2. Activated carbon has a very high added value. Moreover, loss of carbon during this procedure has a major impact on the operating costs due to the high cost of replacing the carbon; PA1 3. Activated carbon becomes saturated with organic, making it necessary to reactivate the carbon with a pyrometallurgical treatment at high temperatures or by using sophisticated chemical processes; PA1 4. The process requires complicated instruments and controls. PA1 1. The technique exhibits low efficiency in removing organic materials, in particular for small concentrations of organic in the feed solution; PA1 2. It does not efficiently remove solids contained in the feed solution; PA1 3. In some specific cases, flotation reagents are used which can subsequently engage in undesirable reactions with the organic phase. PA1 1. Reduced investment costs--By way of example, the investment required for a Degremont filter for the filtration of 100 m.sup.3 /hour is close to $300,000. In the present process, to achieve the same objective the required investment is approximately $60,000; PA1 2. The method of the present invention has substantially lower operating costs than those required for the prior art methods described above; PA1 3. The equipment required for the present invention is very simple in contrast to that utilized in the prior art processes, which means that maintenance costs and costs for spare parts are greatly reduced; PA1 4. The materials of which the filtering bed is formed do not readily deteriorate with extended use. For this reason it does not require constant replenishment or refurbishment as in the case of prior art sand and anthracite filters which become worn out due to abrasion and have to be continually replaced; PA1 5. Significant losses from the filtering bed do not occur in the back used in the present invention as typically happens with other prior art techniques; PA1 6. In the present invention, back washing is required only about once a week during continuous operation. In the case of the prior art Degremont filters, backwashing is typically performed for 2 hours after each 16 hours of operation; PA1 7. Longer intervals between back wash operations involve less consumption of air and water, resulting in lower cost and greater availability and utilization of the equipment; PA1 8. Losses of electrolyte-containing valuable chemicals are substantially eliminated in the present process. In the case of the Degremont filters for the filtration of electrolytes of copper, losses typically reach about 2.5 m.sup.3 per filter for each stage of back wash; PA1 9. The construction materials used in the present invention are low-cost in comparison to the prior art alternatives; PA1 10. The present invention does not require the use of sophisticated instrument and control systems required in prior art systems; PA1 11. The present method in its simplest form operates by gravity and thus does not require pressurization.
A further alternative is the use of after-settlers which, due to their low efficiency, i.e., typically less than 40%, are considered only as a complement to the use of other filtration units. Further, depending on the size of the plant, a significant investment is required both due to the size of the equipment and because of the special materials required for its construction. A 750 m.sup.2 after-settler, for example, requires an investment of about $600,000. Further, the scum floating in these after-settlers has to be removed periodically, which substantially complicates matters when the equipment is large in size.
Flotation techniques, whether carried out in the conventional manner, in columns, or other special designs, are additional alternatives used in the industry. However, these techniques have the following limitations which has caused them to be typically used only as supplementary equipment to the commonly used filters:
Scavenger circuits are normally incorporated in electrowinning plants that utilize solvent extraction to eliminate the organic entrainment. The organic entrainment is eliminated by the use of the well known microflotation technique, which is performed on site with the use of micro-bubbles resulting from the electrochemical reactions. The scavenger circuit thus acts in a manner similar to sacrifice cells to retain the organic entrainment and to ensure the physical and chemical quality of the cathodes of the commercial circuits.
Finally, although filtration and centrifuging techniques are technically feasible alternatives, there are no known commercial applications of these techniques. It is believed that the use of these techniques is limited due to the substantial investment required for constructing and operating systems requiring their use.